This invention relates generally to the field of semiconductor manufacturing, and more particularly to polishing operations in semiconductor manufacturing.
In manufacturing semiconductor integrated circuit components, chemical-mechanical polishing (CMP) can be used to prepare surfaces during fabrication. In the case of shallow trench isolation (STI) techniques, such as illustrated in FIGS. 1A through 1D, a device is generally formed on a silicon substrate 102, with a pad oxide layer 104 and a nitride layer 106 formed thereon. Standard methods, such as chemical or ion etching, can be used to form the shallow trench 108 through the nitride layer 106, pad oxide layer 104 and partially penetrating the substrate 102. The trench 108 can then be backfilled by depositing a fill oxide layer 110 over the nitride layer 106. CMP then can be used to remove the fill oxide layer 110 from the surface of the nitride layer 106, leaving the trench 108 filled and, preferably, the nitride layer 106 substantially intact. Following CMP, the nitride layer 106 can then be selectively removed such that only the pad oxide layer and oxide filling the trench remain. To achieve this result, the CMP process should be terminated before the nitride layer 106 is substantially impinged and before the pad oxide layer 104 is impinged.
FIG. 1E illustrates a non-ideal case where CMP was applied to the structure of FIGS. 1A-1D. In this case, as a result of dishing during the CMP process, the pad oxide layer 104 is penetrated by the polishing process resulting in damage to the active layer of the device, as shown generally by region 112.
There are methods known in the prior art for performing end point detection with CMP. For example, the electrical current required to drive the platen which rotates a polishing pad at a fixed rate, or the current required to drive the wafer carrier, can be monitored to detect the nitride layer 106/pad oxide layer 104 interface. However, because the polishing properties of the nitride layer 106 and pad oxide 104 layer are similar, such methods lack the requisite sensitivity to consistently avoid damaging the active pad oxide layer 104. In addition, as such methods rely on the mechanical properties of relatively large mechanical devices, these processes tend to be too slow to provide precise endpoint detection.
Referring to FIG. 2, it is known that the acoustic and ultrasound resonances of a device depend on the device thickness and geometry. It is also known that as the mechanical properties of a semiconductor device change while undergoing polishing operations, such as CMP, the acoustic properties of the device also change. U.S. Pat. No. 5,222,329 to Yu discloses a system for detecting and controlling CMP using acoustical methods. The system of the ""329 patent uses a microphone to detect acoustical waves in the range of 20-20,000 HZ generated when a wafer is subjected to CMP. The microphone is coupled to a spectrum analyzer which operates to analyze the intensity versus frequency characteristics of the acoustical waves. A CMP computer is coupled to the spectrum analyzer and controls a polishing machine in accordance with the acoustical signals. U.S. Pat. No. 5,245,794 also employs acoustical waves to control a polishing machine. The ""794 patent discloses the use of a transducer sensitive to acoustical waves in the range of 30-100 Hz coupled to an active filter, such as a phase locked loop, to derive control signals for a polisher controller.
Both the ""329 and ""794 patent suffer drawbacks. For example, because low frequency acoustical waves propagate over large distances, low frequency acoustic systems are susceptible to interference from ambient noise. In addition, the large propagation distance of such low frequency acoustic waves makes it difficult to get a local picture of the polishing rate. Therefore, these systems are not able to provide an indication of polishing uniformity.
Accordingly, there remains a need for improved systems and methods of endpoint detection during CMP, especially for shallow trench isolation devices The present invention provides systems and methods that substantially reduces or eliminates problems associated with prior polishing control systems and methods.
In accordance with the present invention, a system for controlling a polishing machine during polishing of a workpiece includes a carrier which has an interface surface for engaging a workpiece, such as a semiconductor wafer, and establishing ultrasonic coupling thereto. At least one crystal oscillator, which has a resonant frequency in an ultrasonic band which is indicative of a polishing depth of the workpiece, is ultrasonically coupled to the carrier. A detector circuit is operatively coupled to the at least one crystal oscillator and provides an output signal which is representative of an output level of the crystal oscillator. A processor circuit is operatively coupled to the detector circuit and provides a signal to the polishing machine which is indicative of a polishing depth, such as a desired polishing endpoint.
More specifically, the carrier can include a vacuum chuck for engaging the carrier to the workpiece. Further, the carrier can be formed with a number of wells therein with a crystal oscillator residing within each of the wells. In a further embodiment, the wells can be filled with an ultrasonic transmission medium to enhance the coupling between the carrier and the crystal oscillator within the wells. In another embodiment, the wells can be arranged in a spatial array about the interface surface of the carrier with a number of oscillators operating at a common resonant frequency residing therein. In such case, the processor can determine a common depth indication at a number of locations on the workpiece.
In an alternate embodiment, the wells can be arranged in a spatial array of well clusters about the interface surface of the carrier with a number of oscillators operating at a number of resonant frequencies which are indicative of a number of different polishing depths at various locations on the workpiece.
A method, in accordance with the present invention, of controlling a polishing machine during polishing of a surface of a workpiece includes the steps of monitoring ultrasonic emissions from a workpiece undergoing polishing by using a crystal oscillator having a resonant frequency; detecting the ultrasonic emissions having a frequency substantially equal to the resonant frequency of the crystal oscillator; and terminating polishing when the ultrasonic emissions have a frequency substantially equal to the resonant frequency of the crystal oscillator.
More specifically, the step of monitoring can include monitoring a number of crystal oscillators which are arranged in a spatial array. The detecting step can include localizing those emissions having a frequency substantially equal to the resonant frequency of the crystal oscillator to a portion of the surface of the workpiece. In addition, the terminating step can then selectively terminate polishing in the portions of the surface of the workpiece where the detected emissions have a frequency substantially equal to the resonant frequency of the crystal oscillator.
In a further method, a number of oscillators can operate at a number of resonant frequencies indicative of a number of different polishing depths and the detecting step can then include steps to determine which of the plurality of polishing depths is indicated. In such a method, before the terminating step, a step of altering the speed of the polishing machine in response to the detected polishing depths can be performed.
A technical advantage of the present invention includes providing systems and methods which employ ultrasound emissions to determine when a desired polishing endpoint has been reached and controlling a polishing operation accordingly. A further technical advantage is that the present systems and methods can detect a desired polishing depth at a number of locations on the workpiece to provide a measure of polishing uniformity. Another technical advantage of the present systems and methods is that a number of polishing depths can be determined at a number of locations and the polishing speed can be controlled accordingly.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.